Twinned columnar growth structure is one of several internal metallurgical structures that can occur in cast alloys when solidification conditions, including grain refining additions are not optimal. Other undesirable features include, for example, columnar grains and residual stress.
Twinned columnar growth (TCG) structure in an aluminum alloy is one of the more deleterious internal structures. It is an anisotropic structure formed as a result of a highly oriented growth pattern and it results in variations of strength, ductility and elasticity of an aluminum alloy. TCG consists of many parallel continuous thin lamellae that are approximately 100 micrometers thick and several centimeters long. Each lamella comprises a twinned crystal with the twinned boundary coherent along the &lt;111&gt; plane and non-coherent at the edges. Growth usually occurs in the &lt;112&gt; direction. The variations of strength, ductility and elasticity of an aluminum alloy due to TCG is a function of the orientation of the growth direction with respect to the tensile testing direction. When the growth direction is parallel with the gauge length of the tensile specimen, strength and ductility is high. When the growth direction is oriented normal to the gauge length, strength is reduced and ductility approaches zero.
Current methods for testing for the presence of internal structural anisotropies, such as TCG, are quite expensive and time consuming. The most commonly employed methods are to scalp or surface machine the ingot, polish and then etch the exposed surface. However, these methods do not test for TCG at a depth below the scalped surface of the metal.
In addition, testing is not performed at multiple levels because multiple transverse section would be needed and there would be less metal for fabrication to a final product. As a result of current testing procedures, TCG may remain undetected until after additional time and capital has been used to work the ingot into a wrought product.
Furthermore, if TCG is detected, current techniques do not allow one to properly quantify it. The TCG found on exposed surfaces is not necessarily representative of the entire ingot. However, it is too costly to test ingots as completely as would be statistically necessary to meet rigid quality control standards. As a result, entire ingots are remelted because TCG is detected in a single portion and there is no inexpensive and nondestructive method to determine if part of the ingot is free of TCG and potentially useful for making quality product.
The present invention is directed to the use of ultrasonic waves to detect, locate and/or quantify structural anisotropies such as TCG throughout the entire cross-section of an ingot. All ultrasonic methods depend in principle upon the fact that the velocity of propagation of ultrasound (elastic waves) in a solid medium is influenced by the state of strain of the medium as well as the elastic constant as a function of direction. Although the effect of TCG on the velocity in an ingot is small, its detection and measurement are within the present state of the ultrasonic art.
However, the velocity of sound is also affected by numerous other factors related to the condition of the material such as its microstructure, heat treatment, grain orientation, density and homogeneity. Therefore, the determination of the absolute velocity of sound in a material does not give an accurate indication of stress in the material unless standards which accurately represent all the other velocity-affecting conditions are available. To overcome this problem of determining the absolute velocity determination, a known technique called shear wave birefringence is used. This technique is based upon measuring the difference in velocity of piezoelectrically generated shear waves which are orthogonally polarized by the electric field applied to the material. According to this technique, only the difference in velocity between two shear waves is measured. Since this difference in velocity is caused primarily by the difference in elastic constants in two orthogonal directions within the material, the need to know the thickness of the metal can be eliminated.
According to the prior art, piezoelectric transducers are required to generate ultrasonic waves in the material being measured. These transducers utilize an oriented crystal which is strained along a particular crystallographic axis in response to an electric field applied to the crystal (the piezoelectric effect). Consequently, the piezoelectric transducer must be rigidly attached or coupled by a very viscous fluid or a solid bond to the material being evaluated in order to inject an ultrasonic wave into the material.
Prior art that discloses the use of ultrasonic waves to determine characteristic of materials are as follows:
U.S. Pat. No. 3,315,520 issued to Carnevale et al discloses an ultrasonic measurement apparatus for determining the ultrasonic transmission characteristics of materials at elevated temperatures. The apparatus utilizes the lapse of time between generating of an ultrasonic wave at the face of the transmitting probe and the arrival of the wave at a second receiving probe to determine the velocity of the wave through the material. The method requires precise thickness measurements to calculate precise velocities. The calculated velocities are then used to determine the temperature of the material.
U.S. Pat. No. 3,504,532 issued to Muenow et al discloses a nondestructive testing system for testing the structural integrity of articles by measuring their sonic characteristics. The system includes a delay circuit which is adjusted until it produces a delay exactly equal to the time required for sound to travel a known distance from a transmitting transducer to a receiving transducer. The delay is read out as an indication of the propagation velocity of waves in the article.
U.S. Pat. No. 4,033,182 issued to Clotfelter discloses a method for measuring biaxial stress in a test articles subjected to stress inducing loads such as engineering structures and the like. The method includes obtaining the transit time differential between a second wave echo for a longitudinal wave propagated along a first path through a stressed test article and the first wave echo for at least one shear wave propagated through the article along a second path paralleling the first path, and then comparing the obtained time differential to establish a transit time differential indicative of a measurement of stress.
U.S. Pat. No. 4,080,836 issued to Thompson et al discloses a method of measuring stress in a material using electromagnetically generated, transverse elastic waves. An electromagnetic transducer is used to generate orthogonally polarized waves traveling through a test block at different velocities as a result of anisotropic stress in the part. The difference in velocity between the polarized waves is measured and compared to the correlation to obtain the stress in the existing part.
In addition, work has been performed using ultrasonic waves to determine characteristic of aluminum plate. See for example, "Effects of Solute Content and Heat Treatment on Elastic Coefficients of Al-Cu Alloys Containing Columnar Crystals", Zairyoshi, Vol 32, No. 352, pages 94-100 in which Yamaguchi et al used ultrasonic sonic wave velocities to determine the elastic stiffness of Al-Cu alloys containing columnar crystals. Yamaguchi et al used great care to prepare the surface of a six inch thick aluminum plate employed to conduct his research. This was done because they were making precise velocity calculations from very precise thickness measurements.
The principal object of the present invention is to provide an inexpensive, nondestructive method for the detection of twinned columnar growth (TCG) in aluminum alloys which is more convenient than prior methods.
Another object of the present invention is to inexpensively detect, locate and quantify structural anisotropies such as TCG throughout the entire cross-section of an ingot in a timely fashion.
Another object of the present invention is to provide a nondestructive method for the detection of anisotropies in as-formed aluminum pieces that is not dependent on making precise velocity measurements.
Still another object of the present invention is to provide a nondestructive method for the detection of anisotropies in as-formed aluminum pieces that is not dependent on making precise thickness measurements to calculate precise velocities.
Another object of the present invention is to provide a nondestructive method for the detection of anisotropies in as-formed aluminum pieces that can utilize rough as formed surfaces and does not require costly surface preparation.
Another object of the present invention is to provide a nondestructive method for the detection of anisotropies in as-formed aluminum pieces having cross-section of approximately two feet.
Yet another object of the present invention is to provide a nondestructive method for the detection of anisotropies in as-formed metals including aluminum alloys without the need to prepare a test surface.
Yet another object of the present invention is to provide a nondestructive method for the detection of anisotropies in as-cast aluminum ingots without the need to scalp the ingot prior to testing.
Additional objects and advantages of the invention will be more fully understood and appreciated with reference to the following description.